Preface

The Z-machine was created on a coffee table in Pittsburgh in 1979. It
is an imaginary computer whose programs are adventure games, and is
well-adapted to its task, implementing complex games remarkably compactly.
They were still perhaps 100K long, too large for the memory of the home
computers of their day, and the Z-machine seems to have made the first
usage of virtual memory on a microcomputer. Further ahead of its time
was the ability to efficiently save and restore the entire execution state.

The design's cardinal principle is that any game is 100% portable to
different computers: that is, any legal program exactly determines its
behaviour. This portability is largely made possible by a willingness to
constrain maximum as well as minimum levels of performance (for instance,
dynamic memory allocation is impossible).

Infocom's catalogue continues to be sold and to be played under
interpreter programs, either original Infocom ones or more recent and
generally better freeware ones. About
130 story files compiled by Infocom's compiler
Zilch survive and since 1993 very many more story files have
been created with the Inform design system.

Eight Versions of the Z-machine exist, and the first byte of any
"story file" (that is: any Z-machine program) gives the Version number
it must be interpreted under.

Standardisation

The opcode names used in this document were agreed between 1994 and 1995
as a standard set by Mark Howell, author of the disassembler Txd
(part of the Ztools suite of utility programs), and Graham Nelson,
author of the assembly level of Inform. They do not correspond to
Infocom's unpublished opcode names.

This Standard was drawn up in November 1995, drawing on a rougher
description written in 1993 and, before that, sketches of table
formats by Mike Threepoint and others. It has formalised
what different interpreter writers regard as the Z-machine,
guaranteeing a reliable and well-featured platform for writers
of new games. The first formal Standard was numbered 0.2, and
this is the second, containing some corrections and clarifications
but also two new features. The following changes are worth noting:

Support for the Unicode character set has been added,
introducing a new table and two new opcodes. S3 has been
rewritten and there are also changes to S7 and S10, as well
as the addition of the opcodes to S14 and S15.

S8.8.3.1, on window attributes in Version 6, has been
rewritten with extensive corrections.

It is now explicit that text buffering never applies
to the upper window in Versions 3 to 5. (In Standard 0.2 the rules
allowed text buffering in Version 4 under some conditions.)

An optional operand (never used and not useful) has
been removed from the opcode set_font. An optional operand,
discovered by Mark Knibbs, has however been added to the Version 6
form of set_colour.

It is now defined that the input character codes for
return and delete are 13 and 8 respectively. (10 and 127 have
been suggested as alternatives in the past).

The fixed-pitch font flag now survives restarts and
restores, like the transcription flag.

Also, the "character set table" is now called the
"alphabet table" (for clarity) and the "mouse data table" has
been renamed the "header extension table."

A companion document to this one, by Martin Frost, defines a standard
format called Quetzal for saved-game files. Standard interpreters
are not required to use Quetzal, since choice of saved-game
format does not affect Z-Machine behaviour, but interpreter-writers
are strongly encouraged to consider it.

Andrew Plotkin has created a standard format called Blorb
for a "resources" file to accompany or encapsulate
a Z-machine game, neatly packaging up sound and graphics in modern
formats. Again, since the Z-Machine has no formal knowledge of the
means of storage of sound or graphics, interpreters are not required
to support Blorb but as a growing number of games are
contained in Blorb files, it is highly recommended that interpreters
support at least basic Blorb capabilities.

So what is "standard"?

To call itself "Standard", an interpreter should (as far as anyone knows)
obey this document exactly for every Version of the Z-machine it claims to
interpret. Interpreters need not provide optional features suggested in
the "remarks" sections, and need not make their source code public.
Each edition of this document has a Revision number, somewhat like the JFIF
identification number used by the JPEG standard. A standard interpreter
should communicate its revision number in three ways:

To someone downloading it from the Internet:
by including it in its filename.

To the player: for instance by means of an "information" option
on a menu, or in an initialisation sequence.

To the game: by writing it into bytes in the header which were
always left zero before this standard was devised (see S11). A game
compiled with Inform library 5/12 or later prints the revision number in its
banner (if this isn't 0.0).

Few arbitrary choices have been made in writing this document.
Where Infocom's own shipped interpreters disagree, or contain manifest
bugs, it has usually been possible to decide which was "correct".
Elsewhere, minimum levels of performance have been
invented where necessary. (For example, a minimum call-stack size
is needed for programmers to be sure of what level of recursion is safe.)

Those few paragraphs which genuinely extend the Infocom format are
marked ***. In any event, Infocom's original shipped interpreters do
not conform to this standard document, because of bugs or because of
slight variations between the Inform output format and Infocom's.

Notation

Hexadecimal numbers are written with an initial dollar, as in $ff,
while binary numbers are written with a double-dollar as in $$11011,
according to Inform conventions. The bits in a byte are numbered 0 to 7,
0 being the least significant and the top bit, 7, the most.

Story files are mechanically best identified by their release number
and serial code, which are written into the header information
at the bottom of Z-machine memory. The release number can be anything
between 0 and 65535 but is usually between 1 and 100.
The serial code can consist of any six textual characters but is
usually the date of compilation, arranged YYMMDD:
thus 970619 refers to June 19th, 1997.

Paul David Doherty, in his extensive investigations into Infocom's
released games, introduced the notation

Release number.Serial code

to identify particular story files: for example the first production
copy of 'Enchanter' is 10.830810. This notation is used throughout
the Standard when individual Infocom files need to be referred to.

Where are all the grammar tables?

The Z-machine has some lexical acuity but it doesn't contain a full parser:
it's like a computer without an operating system. A game program has to
contain its own parser and the tables this uses are not part of the formal
Z-machine specification. (Many Infocom games have similar parsing
table formats simply because, until Version 6, they used an evolving
version of the 'Zork I' parser. A quite different parser was used
in Version 6.) Inform's parsing table formats are documented in the
Inform Technical Manual. For the usual format of Infocom's parsing
tables, see the Ztools utility Infodump.

Acknowledgements

There is an obvious resemblance between an unreadable script
and a secret code; similar methods can be employed to break
both. But the differences must not be overlooked. The code is
deliberately designed to baffle the investigator; the script
is only puzzling by accident.

John Chadwick, The Decipherment of Linear B

The Z-machine was originally devised by Joel Berez and Marc Blank in 1979.
Marc Blank made most of the Version 4 extensions, and Version 5 was created
by Dave Lebling (with contributions from others including Brian Moriarty,
Duncan Blanchard and Linde Dynneson). Version 6 was largely the work of Tim
Anderson and Dave Lebling.

In the reverse direction, decipherment is mostly due to the InfoTaskForce
(David Beazley, George Janczuk, Peter Lisle, Russell Hoare and Chris Tham),
Matthias Pfaller, Mike Threepoint, Mark Howell, Paul David Doherty and
Stefan Jokisch. Only a few of the pieces in the jigsaw were placed by
myself.

I gratefully acknowledge the help of Paul David Doherty and Mark Howell, who
each read drafts of this paper and sent back detailed corrections; also, of
Stefan Jokisch and Marnix Klooster who have put a great deal of work into
the fine detail of the specification; and of all those who commented on
the circulated draft. Mistakes and misunderstandings remain my own.

Graham Nelson

15 November 1995

Kevin Bracey and Stefan Jokisch discovered most of the mistakes in
Standard 0.2, in developing the first Version 6 interpreters of the
modern age: Zip2000 and Frotz. Matthew Russotto
and Mark Knibbs supplied helpful information about Infocom's own
Version 6 interpreters. Stefan also kindly read and commented on
numerous drafts of the present revision. Finally, discussion about
this document was greatly assisted by the Z-Machine Mailing
List, organised by Marnix Klooster.

Graham Nelson

22 June 1997

The majority of the clarifications and updates in this latest revision
are the work of Kevin Bracey and Jason C. Penney. Thanks go also to the
members of the (now defunct) Z-Machine Mailing List, and those of the
intfiction.org forum, especially Dannii Willis, for bringing to light
issues with my initial revision. Special thanks to Andrew Plotkin
for his notes, advice and general help while working on this revised document.